Kettle souring is a process that allows beer brewers to consistently whip up batch after batch with the perfect level of tartness. Unlike traditional souring methods, which can take months or even years, kettle souring can be pulled off in as little as 24 hours. Starting with a basic wort, add a strain of pure lactobacillus and give the bacteria enough time to start breaking down the sugars in the liquid. Once it reaches your preferred pH level, you’ll be left with a brew that’s light, crisp, and refreshingly tangy.

Fill your brewing kettle with water. Start with freshly purified water, making sure it’s completely clean, clear and odorless. A good rule of thumb is to run about 1.5 quarts (1.41L) of water for every pound of malt you plan on using.[1]XResearch source

The pH balance and mineral content of your water can affect the flavor of the finished brew, so try to find a good neutral source.

You can contact your local municipal officials to get a full report about the contents of your city’s water supply.[2]XResearch source

Heat the water to 165°F (74°C). Turn on the burner beneath the kettle and let it begin warming up. If the kettle you're using doesn't have a direct heat source, boil the water first before filling the container and let it allow it to cool to the appropriate temperature.[3]XResearch source

The malt extract you’ll be using to make the wort will have an easier time dissolving in higher temperatures.[4]XResearch source

Wort is the liquid left over from the mash that contains the sugars needed to begin the souring process.

Add the malt extract. Stir in the powdered malt slowly, making sure to break up any large, doughy clumps that happen to form on the surface. Continue stirring the mixture until the malt is completely dissolved and distributed evenly throughout the tun.

Most homebrew starter kits include malt extract among their ingredients.

A basic malt extract is the simplest way to prepare a wort for sour kettling. As your brewing skills, however, you can begin experimenting with other mash-making methods, like milling your own special grains.

Let the wort sit for one hour. As the wort rests, the malt extract will begin releasing its natural sugars. Give the mixture an occasional stir during this time—otherwise, keep it covered.[5]XResearch source

To test whether your wort has had ample time to absorb the sugars from the malt, perform the trusty iodine test. Cool off about an once of wort and add a couple drops of iodine. If the liquid turns a dark purple color, it's not quite ready. If there is no change in color, it means the most of the starches have already dissolved.[6]XResearch source

If you prefer a more potent brew, feel free to tack on an additional 15 to 30 minutes.

Keep the wort at a consistent temperature. After adding the malt extract, the water temperature should remain between 148-154°F degrees (64-68°C). If the wort cools off too much, briefly turn up the heat or add a small amount of boiling water until it reaches the desired temperature once more.[7]XResearch source

Try not to let the temperature of the wort drop below about 85°F (30°C). Doing so may result in a watery wort without much flavor.

Keep the kettle insulated using a canvas tarp, blankets or similar items.

Boil the wort for at least 5 minutes. A quick preliminary boil will help sterilize the wort by killing off bad bacteria, enzymes and other unwanted substances. If left untreated, these byproducts could potentially interfere with the finished flavor of the beer, or even make you sick.[8]XResearch source

This first boil can last as long as 10-15 minutes, depending on the size of the batch you’re producing.

Be sure to also sterilize any other utensils that have come into contact with the wort.

Cool the wort to around 100°F (37°C). Turn down the burner or partially uncover the kettle to allow heat to escape. Lactobacillus bacteria prefer a warm environment, so make sure you’ve reduced the wort to a hospitable temperature before you introduce the cultures.[9]XResearch source

It’s not necessary to keep the wort at exactly 100°—lactobacillus can thrive in temperatures as low as 85° (30°C). However, lower temperatures will cause the souring process to take longer.

Bring the wort to a pH level of 4.5. Add a few drops of food grade lactic or phosphoric acid to the kettle and stir, then use a pH meter to test the acidity. Starting with a baseline acidity will create the most beneficial fermentation conditions possible and help the bacteria to do its thing much faster.[10]XResearch source

Balancing the pH of your wort prevents other strains of bacteria from taking over and making the beer dangerous or unpleasant to drink. It also protects the proteins in the yeast, which means your beer will turn out with a rich head and plenty of body.

A handheld dropper tool will give you more control and prevent you from adding too much acid at once.

Introduce the lactobacillus to the wort. Dump the cultures straight into the kettle, then stir thoroughly and cover. In order to successfully sour a batch of beer, you’ll need to add approximately 10 million lactobacillus cells for every mL of wort. Look closely at the measurements listed on the packaging to calculate exactly how much you’ll need for the amount of wort you’re working with.[11]XResearch source

Most brewmasters recommend using a pure culture, as these tend to produce more consistent and predictable results.[12]XResearch source

Pure cultures of bacterial additives like lactobacillus are often sold in small vials at homebrew supply stores. If you’re having trouble tracking these down, one useful alternative may be to look for pure cultures in the form of dietary supplements.

Wait for the souring process to take place. In most cases, this will take somewhere between 24 and 48 hours. Of course, the size of your batch will have an effect on the overall souring time. Come back and check the wort’s progress every 8-12 hours.[13]XResearch source

As the wort sits, the lactobacillus will feast on the sugars in the liquid, producing lactic acid as a waste product. This lactic acid is what gives sour beers their characteristic notes.

Test the acidity of the mash. The best way to do this is to use a reliable pH meter. You’ll be looking for a pH of around 3.6 or higher to replicate mildly tart styles like Berliner Weisse, Gose and most saisons. Acidity levels closer to 3.3 will make for beers with flavors that resemble young lambics and traditional sours.[14]XResearch source

The lower the pH, the more acidic (and therefore more sour) the beer will be.

If you don’t have a pH meter, you can test the tartness of the wort the old fashioned way by tasting it—just be sure the utensil you’re using is germ-free.

Boil the wort for 60-90 minutes. Once you’ve achieved the desired sourness, you can cook up the wort as usual. A second, longer boil is necessary to stabilize the remaining bacteria for a smoother flavor and body. Feel free to incorporate your choice of hops and other additives at this point.[15]XResearch source

For a more festive-tasting brew, try adding different combinations of fresh fruits or flavorful spices.[16]XResearch source

Add yeast to complete the fermentation process. Chill the water around the kettle to bring the temperature down, then shake in your chosen yeast in the correct proportion and stir. Afterwards, transfer the wort to a fermentation container, seal it up, and let the waiting game begin.[17]XResearch source

Keep experimenting until you hit on the perfect balance of flavor and acidity. Brewing is like chemistry—it requires a lot of trial and error.

Allow the wort to ferment for 1-2 weeks. In a matter of days, you’ll have a batch of delicious homemade beer with just the right amount of sourness. During this time, be sure to store your wort somewhere cool and dark. Basements and garages are ideal for storing homebrews while they finish undergoing fermentation.

Use your own sensibilities as a connoisseur to help you decide when a particular batch has had enough time to ferment. The longer you let it rest, the more pronounced the sharp, pungent flavors will be.

Kettle souring is perfect for brewers who want to enjoy the zip of high-acidity beers but don't want to spend countless months waiting for the cultures from raw grains to work their magic.[18]XResearch source

Tips

Eyeballing and estimating usually results in undrinkable beer. Rely on essential tools, like your thermometer, pH meter, temperature gauges, and hydrometer to take accurate readings every step of the way.

If you’re unable to get your hands on a pure culture of lactobacillus, try making your wort with yogurt. It may sound strange, but plain yogurt contains the same bacterial cultures as milled grains, and can make a convenient solution in a pinch.